refactor(manifold): one applyCurve, one GROUP_COLOR — and make the knob honest

Phase 3 (L38, ST4).

L38 is a REAL, VISIBLE behaviour change, and it is a bug fix rather than a wash.
There were two divergent applyCurve implementations: model.ts's
(`e = 0.25 + c*1.75`, linear at c≈0.43) drove the on-screen bars, while
mapping.ts's spec-anchored version (0.5 = exactly linear, symmetric in
log-exponent space) drove `mapOutput` — i.e. the value actually sent to MIDI/OSC/
VCV. So the display disagreed with the signal at every curve setting except the
shared endpoint. mapping.ts's survives; model.ts's is deleted and re-exported.
Displayed values now move visibly, and they now match what is transmitted.

That unification exposed a third copy the finding did not mention: CurvePad.tsx
inlined the same deleted formula to draw the curve-knob preview and its numeric
readout. Left alone it would have matched NEITHER implementation — the knob
would have drawn one response while the bars and the output used another.
CurvePad now calls the same applyCurve, so knob, bars and transmitted value are
one thing. The readout shows the actual exponent.

ST4: GROUP_COLOR was duplicated across four files (Phase 1's ReadoutStrip
deletion removed a fifth). All four now import the single map in model.ts —
including OutputStage.tsx's OUT_GROUP_COLOR, which was outside the sweep's scope
and would have been the last survivor.

NOT done, deliberately: ST4 also invited replacing the fixed map with a
hash-to-palette function, because its keys (formant/pitch/amp/filter/fx/mod) are
JSX-era placeholders that do not match the real schema group strings
(verb/sequencer/general/filterbank/delays/eq/midi/...), so most groups currently
fall through to the accent colour. That is a visual redesign, not a
deduplication, and it should be one deliberate change now that every consumer
reads one map.

Gates: typecheck clean, 17/17 unit tests, 33 e2e specs.

manifold/src/console/CompositeStage.tsx

@@ -22,6 +22,7 @@ import { VirtualJoystick, XYPad } from '../primitives';
 import { Manifold } from './Manifold';
 import { OutputStage } from './OutputStage';
 import { MiniMeters } from './shared-ui';
+import { GROUP_COLOR } from './model';
 import type { MFMode, MFParam } from './model';
 import type { FeedbackMarker, Pin } from './types';
 
@@ -29,15 +30,6 @@ const SNAPS = [0.14, 0.33, 0.5, 0.66, 0.86];
 const MAGNET = 0.026;
 const SHUT = 0.1;
 
-const GC: Record<string, string> = {
-  formant: '--accent',
-  pitch: '--accent-2',
-  amp: '--good',
-  filter: '--warn',
-  fx: '--info',
-  mod: '--accent-3',
-};
-
 const CORNERS: Record<string, CSSProperties> = {
   tl: { top: 62, left: 14 },
   tr: { top: 62, right: 14 },
@@ -261,7 +253,7 @@ export function CompositeStage({
       >
         {params.map((p, i) => {
           const eff = values[i] ?? 0;
-          const gc = `var(${GC[p.group] || '--accent'})`;
+          const gc = `var(${GROUP_COLOR[p.group] || '--accent'})`;
           const dim = p.status === 'off';
           const set = (cx: number, el: HTMLDivElement) => {
             const r = el.getBoundingClientRect();

manifold/src/console/CurvePad.tsx

@@ -1,9 +1,14 @@
 /**
  * CurvePad — a square response-curve plot. Vertical drag reshapes the curve.
- * `curve` is 0..1 where ~0.43 reads as linear; mirrors the engine's applyCurve.
+ * `curve` is 0..1 where 0.5 is exactly linear.
+ *
+ * The preview draws the SAME `applyCurve` the output path uses
+ * (`backends/mapping.ts`) rather than re-deriving an exponent, so the knob, the
+ * on-screen bars and the value actually sent to a backend can never disagree.
  * Ported from `CurvePad.jsx`.
  */
 import { useEffect, useRef } from 'react';
+import { applyCurve } from '../backends/mapping';
 import type { PointerEvent as ReactPointerEvent } from 'react';
 
 export interface CurvePadProps {
@@ -48,14 +53,13 @@ export function CurvePad({ curve = 0.5, onChange, size = 116 }: CurvePadProps) {
     ctx.setLineDash([]);
     const css = getComputedStyle(cv);
     const accent = css.getPropertyValue('--accent').trim() || '#ff6a00';
-    const e = 0.25 + curve * 1.75;
     ctx.strokeStyle = accent;
     ctx.lineWidth = 2;
     ctx.beginPath();
     const pad = 3;
     for (let p = 0; p <= 80; p++) {
       const xv = p / 80;
-      const yv = Math.pow(xv, e);
+      const yv = applyCurve(xv, curve);
       const px = pad + xv * (size - 2 * pad);
       const py = size - pad - yv * (size - 2 * pad);
       if (p === 0) ctx.moveTo(px, py);
@@ -95,7 +99,7 @@ export function CurvePad({ curve = 0.5, onChange, size = 116 }: CurvePadProps) {
         <span
           style={{ fontSize: 10, color: 'var(--fg-dim)', fontVariantNumeric: 'tabular-nums' }}
         >
-          {(0.25 + curve * 1.75).toFixed(2)}
+          {Math.pow(2, 4 * (curve - 0.5)).toFixed(2)}
         </span>
       </div>
       <canvas

manifold/src/console/OutputStage.tsx

@@ -6,16 +6,9 @@
 import { useRef, useState } from 'react';
 import type { PointerEvent as ReactPointerEvent } from 'react';
 import type { MFParam, ParamStatus } from './model';
+import { GROUP_COLOR } from './model';
 import { OutputEditor } from './OutputEditor';
 
-const OUT_GROUP_COLOR: Record<string, string> = {
-  formant: '--accent',
-  pitch: '--accent-2',
-  amp: '--good',
-  filter: '--warn',
-  fx: '--info',
-  mod: '--accent-3',
-};
 const OUT_NEXT: Record<ParamStatus, ParamStatus> = { off: 'fixed', fixed: 'live', live: 'off' };
 
 export interface OutputStageProps {
@@ -94,7 +87,7 @@ export function OutputStage({ params, values, onChange, compact = false }: Outpu
     >
       {params.map((p, i) => {
         const eff = values[i] ?? 0;
-        const gc = `var(${OUT_GROUP_COLOR[p.group] || '--accent'})`;
+        const gc = `var(${GROUP_COLOR[p.group] || '--accent'})`;
         const dim = p.status === 'off';
         const placeRight = i > params.length - 4;
         return (
@@ -119,7 +112,7 @@ export function OutputStage({ params, values, onChange, compact = false }: Outpu
                   color:
                     p.status === 'live'
                       ? 'var(--fg-mute)'
-                      : `var(${OUT_GROUP_COLOR[p.group] || '--accent'})`,
+                      : `var(${GROUP_COLOR[p.group] || '--accent'})`,
                   overflow: 'hidden',
                   whiteSpace: 'nowrap',
                   textOverflow: 'ellipsis',

manifold/src/console/shared-ui.tsx

@@ -6,17 +6,9 @@
  * Manifold ships a single "composite" altitude). MiniMeters survives — it is
  * the live glanceable output readout used by CompositeStage's minimap.
  */
+import { GROUP_COLOR } from './model';
 import type { MFParam } from './model';
 
-const MM_GROUP_COLOR: Record<string, string> = {
-  formant: '--accent',
-  pitch: '--accent-2',
-  amp: '--good',
-  filter: '--warn',
-  fx: '--info',
-  mod: '--accent-3',
-};
-
 /** MiniMeters — glanceable read-only output bars (no interaction). */
 export function MiniMeters({ params, values }: { params: MFParam[]; values: number[] }) {
   return (
@@ -41,7 +33,7 @@ export function MiniMeters({ params, values }: { params: MFParam[]; values: numb
               right: 0,
               bottom: 0,
               height: `${v * 100}%`,
-              background: `var(${MM_GROUP_COLOR[params[i]?.group] || '--accent'})`,
+              background: `var(${GROUP_COLOR[params[i]?.group] || '--accent'})`,
               opacity: 0.3 + v * 0.6,
             }}
           />

manifold/src/dock/OutputControlRow.tsx

@@ -9,6 +9,7 @@
  */
 import { useRef } from 'react';
 import type { CSSProperties, PointerEvent as ReactPointerEvent } from 'react';
+import { GROUP_COLOR } from '../console/model';
 import type { MFParam, ParamStatus } from '../console/model';
 import { CurvePad } from '../console/CurvePad';
 
@@ -18,15 +19,6 @@ const STATE_META: { v: ParamStatus; label: string; color: string }[] = [
   { v: 'live', label: 'live', color: 'var(--accent)' },
 ];
 
-const GROUP_COLOR: Record<string, string> = {
-  formant: '--accent',
-  pitch: '--accent-2',
-  amp: '--good',
-  filter: '--warn',
-  fx: '--info',
-  mod: '--accent-3',
-};
-
 /** A compact dual-thumb min/max range (min blue, max orange — dock-spec §3.1). */
 function DualRange({
   min,